1. Field of the Invention
The present invention relates generally to a hydraulic cylinder assembly for an automotive suspension system for controlling suspension characteristics depending upon vehicle driving condition. More specifically, the invention relates to an actively controlled suspension system with a hydraulic cylinder assembly.
2. Description of the Background Art
In the recent years, actively controlled suspension systems have been developed and proposed. Such an actively controlled system includes a hydraulic cylinder assembly interposed between a vehicle body and a suspension member which rotatably support a vehicular wheel. The hydraulic cylinder assembly is connected to a working fluid pressure source via a hydraulic circuit. For example, such an actively controlled suspension system has been disclosed in the U.S. Pat. No. 4,702,490, issued on Oct. 27, 1987.
In such actively controlled suspension systems, there is a proposal for improving suspension control characteristics and response characteristics. Such improvement has been disclosed in the European Patent First (unexamined) Publication No. 02 49 209. In the disclosed system, a pressure accumulator is connected to the hydraulic cylinder assembly for accumulating excessive hydraulic pressure and supplying hydraulic pressure to the hydraulic cylinder. Since the accumulator is directly in communication with a pressure chamber of the hydraulic cylinder, it will provide capacity for the hydraulic cylinder to absorb road shock by adjusting hydraulic pressure and to provide sufficiently high response characteristics by quick supply and quick drain of pressurized fluid.
The pressure accumulator, proposed in the aforementioned European Patent First Publication No. 02 49 209, a pneumatic pressure chamber and a hydraulic pressure chamber separated by an elastic diaphragm member is included. The diaphragm member is made of rubber. When such a rubber diaphragm is subjected to high temperature, leakage of gas in the pneumatic pressure chamber tends to occur to lower pressure accumulation capacity. Furthermore, since such a rubber diaphragm is frequently subject to pressure variation caused due to road shock, secular variation in the diaphragm tends to occur in a relatively short period.